1. Field of the Invention
The present invention relates to an image reading apparatus for optically reading an image on an original.
2. Description of the Related Art
In general, high speed, high resolution, high image quality, wide-width (A3 size) reading, and the like are required for a digital-type image reading apparatus, and a countermeasure against the lifting up of an original is required in the case of a copying machine or the like in light of the fact a booklet folded in two, a thick book, or the like is used as a original. If these requirements are considered in physical and optical terms, a high Nyquist frequency, a high modulation transfer function (MTF) characteristic, and high sensitivity, a wide field angle, and a deep focal depth of the sensor are required. As items which concern the specifications of an image-forming optical system after the determination of the sensor characteristics and conjugate length among the aforementioned items, it is possible to cite the height of the MTF and the focal depth.
Further, in the case of a color copying machine, a color scanner, and the like, the chromatic aberrations of the lens make an issue in addition to the aforementioned conditions. The chromatic aberrations include the longitudinal chromatic aberration which represents the difference in the position of the image point on the optical axis from one color to another as well as the chromatic aberration of magnification which represents the difference in the image size, and these chromatic aberrations respectively exert different influences on the evaluation of the image-forming optical system. Specifically, the longitudinal chromatic aberration offsets the peak of the MTF of each color in the image-forming optical system in the defocusing direction, and because it is necessary to establish a balance thereof, the longitudinal chromatic aberration deteriorates the MTF and results in the aggravation of the image quality. In contrast, since the chromatic aberration of magnification offsets the image-forming position for each color in the longitudinal direction of the charge coupled device (CCD) sensor, color offset occurs as a result.
As for the MTF characteristic, other than the chromatic aberrations the aberration of a single color also deteriorates it, so that its mechanism is very complicated. In addition, included among items of evaluation concerning the MTF are the height of the MTF at the CCD sensor surface, the allowable range (focal depth) in the case where the original is lifted up, and the color balance (DMTF) at each field angle. Among them, an increase in the DMTF and an increase in the chromatic aberration of magnification, although different in physical (optical) behavior, produce similar results in the light of an output from the CCD sensor. Namely, signal values to the outputted from the tricolor pixels at the same position in the longitudinal direction of the sensor change, with the result that a phenomenon occurs in which color information which combines them also changes from its original value. The fact that the color information changes means that the color reproducibility of an output image with respect to the original is deteriorated in a color copying machine. In the case of the color copying machine, there is a step in which the information which is read in blue (B), green (G), and red (R) is transformed into yellow (Y), magenta (M), cyan (C), and black (K) which are the original colors of the toner, but if the original data which has been read is offset by unspecified amounts, correction is difficult.
Further, coloring of edges in a region of the same color is among defects peculiar to the chromatic aberration of magnification. In the case of copying and outputting an image whose contour is often unclear as in the case of a photograph, slight color offset is often neglegible. However, in the case of an original which is composed by combining characters and graphic figures as in the case of a business document, if only edge portions of the characters and graphic figures in the horizontal scanning direction are set in a different color due to the chromatic aberration of magnification, that color becomes very noticeable, and leaves an unfavorable impression on looking at it. In contrast, ni the case of the MTF, although a difference in color appears overall, the situation does not occur in which only edge portions assume a different color. In this respect, the chromatic aberration of magnification rather than DMTF is difficult to cope with.
Furthermore, in the case of a digital color copying machine, image processing generally differs depending on whether the original is a text or an image such as a photograph. In the case where the original is a text, clear contrast between the text and the background facilitates reading. For this reason, in the case of the text original, image processing for enhancing edge portions of the text is effected. In the case of the image original, in contrast to the aforementioned text original, an unsharp change in color leaves a favorable impression on looking at it. Since the manner of image processing is thus changed depending on whether the original is a text or an image, most image processing units of copying machines have separate processing functions for text/image (T/I) built into them.
In addition, if the color which has been read is black or gray, a beautiful output can be obtained by expressing the color with a K toner instead of expressing it by superimposing Y, M, and C toners. For this reason, many color copying machine are provided with black-color determination processing functions in addition to the aforementioned separate processing functions for T/I. At that time, when a black color is separated from image information, if the output from the CCD sensor is offset from black due to the aforementioned aberration, a large problem occurs. If a T/I separation processing error and a black-color determination error occur at the same time, black character portions of the original are outputted in a colored and blurred state, with the result that a user is left an impression that the output is quite different from the original.
Particularly in recent years, full-color originals including high-resolution character information have begun to flood due to the development and widespread use of personal computers and presentation-material preparing tools, and the frequency at which such originals are copied has also increased. For this reason, defects ascribable to the aforementioned chromatic aberrations and DMTF have conceivably come to occur frequently.
In addition, in an image reading apparatus for reading a wide (A3-size) original at a high speed, to increase the sampling pitch of the reading sensor, i.e., the reading resolution, from 400 dpi to 600 dpi, it is necessary to reduce the pixel size of the sensor, so that the sensor sensitivity drops to a half or less. With respect to this problem, although increased sensitivity has been attained by improvement of the S/N ratio of the CCD sensors in recent years, in order to increase the reading resolution from 400 dpi to 600 dpi, as described above, and maintain a reading velocity equivalent to the case of 400 dpi, it is necessary to secure the brightness of F3.5 to F4.5 or thereabouts as the image forming lens assuming that electric power for illumination for illuminating the original with light is fixed.
Meanwhile, in order to read a large-size original, the field angle of the image forming lens must be increased. For example, if an A3-size original is to be read, a lens design is required which covers the reading range at a half field angle of 180 to 200. In addition to such a requirement, in the color image reading apparatus, the chromatic aberration of magnification for color image processing needs to be reduced. However, if an attempt is made to strictly incorporate such a requirement, other aberrations such as the curvature of field tend to become large, so that the lens design is made more difficult.
If under the condition of the ordinary cost the design is made to satisfy the aforementioned various conditions, i.e., to make the F-value of the lens small, enlarge the field angle, and reduce the chromatic aberration of magnification, the resolution in the radial direction of the lens declines. The decline in this resolution is noticeable in the case of the light which is remote from the optical axis of the lens, i.e., the reflected light from an end portion of the original. Since the resolution in the radial direction affects the reading performance in the vertical scanning direction of the reading sensor, the resolution of peripheral portions of the image will decline under the above-described conditions. Additionally, if the best image plane is curved, there occurs the problem that, coupled with the longitudinal chromatic aberration of the lens, offset (difference in MTF) in the modulation of the three colors of red (R), green (G), and blue (B) occurs. For instance, when a black character is read, the lines of the black character become colored, and the image processing unit is unable to recognize it as the black character, thereby deteriorating the character quality.
Therefore, as a measure for overcoming such a problem, a means is conventionally adopted in which the lens is made dark to correct the aberrations. However, if an attempt is made to correct the chromatic aberration of magnification by the lens design, the MTF characteristic tends to deteriorate due to the curvature of field. For this reason, even if anomalous dispersion glass, which is effective as a measure against chromatic aberrations, is used, the improvement of the characteristic of the lens proper is naturally limited. In addition, since the anomalous dispersion glass is three to five times higher in cost than ordinary glass materials, the use of the anomalous dispersion glass involves a substantial increase in cost.
In addition, to reduce the aberrations at the lens designing stage, a technique is generally adopted in which the aperture is reduced or the field angle is reduced. However, the field angle cannot be changed substantially since a constraining condition is imposed by the size of the original and the space of the copying machine. Therefore, a technique is often adopted in which the aperture is reduced (the lens is made dark) to reduce the aberrations. However, if the lens is made dark while maintaining a high speed and a high resolution, it is necessary to increase the amount of light from the lamp (light source) to secure a necessary amount of light in the light of the S/N ratio of the sensor, resulting in an increase in power consumption. Further, to realize the high speed, high resolution, and high quality as described above, the amount of exposure on the sensor surface cannot be allowed to decline. For this reason, to secure an necessary amount of exposure, it is necessary to increase the amount of light for illumination for illuminating the original with the light. However, since the increase in the quantity of illuminating light directly leads to an increase in power consumption, such a measure is not advantageous for reasons state below.
Namely, in the color image reading apparatus, a linear halogen lamps is frequently used because it excels in reproducibility of colors which have been read and because a large amount of light can be obtained. However, since this lamp is inferior in the efficiency for converting electric power to light, power consumption becomes inevitably large. In contrast, although working electric power per power-supply system which can be supplied from a power supply outlet in an ordinary office environment is 1.5 kVA, in the case of an electrophotographic copying machine, very large electric power is required by a heater portion for generating heat since heat is used for fixing the toner. In addition, even in the case of reading with 400 dpi in the current state, power consumption by the copying machine is a value which is narrowly lower than 1.5 kVA. For this reason, electric power which is consumed by the original reading unit needs to be reduced as practically as possible.
Further, in the case where an image is subjected to read scanning for each line by the line sensor, the effect of flicker when the halogen lamp is AC lit at a commercial frequency cannot be ignored. In particular, when high-gradation reading is effected in color, even in the case of a thermal light source such as a halogen lamp which produces a relatively small amount of flicker, the quality of the image which has been read is affected by the flicker. For this reason, in order to effect favorable reading, there arises the need to convert the power from AC to DC and DC light the halogen lamp. However, since a power loss occurs if this AC/DC conversion is effected, power which is consumed for illumination becomes increasingly large.
In addition, if an attempt is made to increase the reading resolution, since there is an increase in power consumption due to an increase in the video rate in the image processing unit, leeway in electric power which can be accommodated to an increase in the amount of illuminating light is equivalent to practically nil. For this reason, the securing the amount of exposure by increasing the amount of illuminating light in the above-described manner is not advisable if resultant disadvantages of the overall copying system (e.g., an increase in the standby time due to warming-up and a decline in the processing speed) are taken into account.
With respect to this problem, a technique has been conceived in which, by making use of inexpensive memories since the prices of memories have dropped in recent years, information which has been read is temporarily stored, and the reading speed is delayed in asynchronism with the copying output side (printer side), or the speed of reading in a single stroke is delayed by effecting full-color reading by single-stroke scanning. However, if this technique is used, the black-and-white copying time tends to be delayed in a color copying machine, so that it is necessary to strike a balance with the color copy. Nevertheless, strictly speaking, the ideal lies in increasing the reading speed irrespective of the black-and-while and color.
There is another reason for designing the lens to be as bright as possible. It is because, as a recent trend of a black-and-white copying machine, a xenon lamp with low power consumption has come to be used as the original illuminating lamp instead of the halogen lamp, and in the case of the full-color copying machine as well, a move to adopt the xenon lamp can be estimated from the standpoint of energy saving. Under the present circumstances, however, since the xenon lamp is inferior to the halogen lamp in the capability of outputting the amount of light, only data with a poor S/N ratio can be read by a dark lens.
Further, as a measure against chromatic aberration of magnification, a technique for electrically correcting the center of gravity with respect to an output of a CCD sensor is also known. However this measure leads to deterioration of the MTF as an electrical signal, and adversely affects the reproduction of characters. Consequently, even if the resolution is increased with much effort, there are apprehensions that that effort merely remains as a fact in the catalog specifications, and a situation can occur in which insofar as the result of output is actually observed, no improvement in the image quality can be recognized. Further, in a case there the image formation characteristic which cannot be covered by the lens is corrected by an electrical circuit or other optical element, such a case positively leads to higher cost if the cost incidental to it is considered.
Against such a backdrop, as a means for improving the resolution in the vertical scanning direction where problems occur, a sagittal stopper (light shielding plate) for condensing the lens aperture in the vertical scanning direction (in the sagittal direction of the lens) is in use.
Conventionally, however, after the image forming lens is designed with a good balance as much as possible so as to obtain a fixed level of resolution or higher over the entire region of the original in the horizontal scanning direction, and the sagittal stopper is used only as an auxiliary means if the required specifications cannot be still satisfied. Therefore, it cannot necessarily be said that the characteristics of the image-forming optical system as a whole have been set in the best state.